A hemodynamic load in vivo induces cardiac expression of the cellular oncogene, c-myc

Pyruvate Kinase M2: A Potential Regulator of Cardiac Injury Through Glycolytic and Non-glycolytic Pathways

ABSTRACT

Adult animals are unable to regenerate heart cells due to postnatal cardiomyocyte cycle arrest, leading to higher mortality rates in cardiomyopathy. However, reprogramming of energy metabolism in cardiomyocytes provides a new perspective on the contribution of glycolysis to repair, regeneration, and fibrosis after cardiac injury. Pyruvate kinase (PK) is a key enzyme in the glycolysis process. This review focuses on the glycolysis function of PKM2, although PKM1 and PKM2 both play significant roles in the process after cardiac injury. PKM2 exists in both low-activity dimer and high-activity tetramer forms. PKM2 dimers promote aerobic glycolysis but have low catalytic activity, leading to the accumulation of glycolytic intermediates. These intermediates enter the pentose phosphate pathway to promote cardiomyocyte proliferation and heart regeneration. Additionally, they activate adenosine triphosphate (ATP)-sensitive K + (K ATP ) channels, protecting the heart against ischemic damage. PKM2 tetramers function similar to PKM1 in glycolysis, promoting pyruvate oxidation and subsequently ATP generation to protect the heart from ischemic damage. They also activate KDM5 through the accumulation of αKG, thereby promoting cardiomyocyte proliferation and cardiac regeneration. Apart from glycolysis, PKM2 interacts with transcription factors like Jmjd4, RAC1, β-catenin, and hypoxia-inducible factor (HIF)-1α, playing various roles in homeostasis maintenance, remodeling, survival regulation, and neovascularization promotion. However, PKM2 has also been implicated in promoting cardiac fibrosis through mechanisms like sirtuin (SIRT) 3 deletion, TG2 expression enhancement, and activation of transforming growth factor-β1 (TGF-β1)/Smad2/3 and Jak2/Stat3 signals. Overall, PKM2 shows promising potential as a therapeutic target for promoting cardiomyocyte proliferation and cardiac regeneration and addressing cardiac fibrosis after injury.

A PKM2 signature in the failing heart

A salient feature of the failing heart is metabolic remodeling towards predominant glucose metabolism and activation of the fetal gene program. Sunitinib is a multitargeted receptor tyrosine kinase inhibitor used for the treatment of highly vascularized tumors. In diabetic patients, sunitinib significantly decreases blood glucose. However, a considerable proportion of sunitinib-treated patients develop cardiac dysfunction or failure. We asked whether sunitinib treatment results in shift towards glycolysis in the heart. Glucose uptake by the heart was increased fivefold in mice treated with sunitinib. Transcript analysis by qPCR revealed an induction of genes associated with glycolysis and reactivation of the fetal gene program. Additionally, we observed a shift in the enzyme pyruvate kinase from the adult M1 (PKM1) isoform to the fetal M2 (PKM2) isoform, a hallmark of the Warburg Effect. This novel observation led us to examine whether a similar shift occurs in human heart failure. Examination of tissue from patients with heart failure similarly displayed an induction of PKM2. Moreover, this phenomenon was partially reversed following mechanical unloading. We propose that pyruvate kinase isoform switching represents a novel feature of the fetal gene program in the failing heart.

TGF-beta 1 and fibroblast growth factors selectively up-regulate tissue-specific fetal genes in cardiac muscle cells

TGF-beta 1, like basic and acidic fibroblast growth factor (FGF), inhibits differentiated gene expression in skeletal myoblasts. It potentiates FGF-beta 1 down-regulated expression of the alpha-myosin heavy chain gene and the sarcoplasmic reticulum calcium ATPase gene, yet up-regulated expression of the genes for beta-myosin heavy chain, atrial natriuretic factor, and both skeletal and smooth muscle alpha-actin-four transcripts associated with the embryonic heart. TGF-beta 1 did not affect cardiac alpha-actin gene expression. These responses resemble the generalized 'fetal' phenotype seen during hypertrophy triggered by a haemodynamic load. Chick skeletal and cardiac alpha-actin promoter-driven reported genes were transfected into neonatal rat cardiac myocytes. TGF-beta 1 stimulated skeletal alpha-actin transcription, but not transcription from the cardiac alpha-actin promoter. Basic FGF produced the same results as TGF-beta 1, but acidic FGF suppressed expression of both alpha-actin genes; these results were true for purified and recombinant FGFs. Modulation of alpha-actin transcription by growth factors corresponded accurately to control of the endogenous genes. Three positive cis-acting elements were critical for skeletal alpha-actin transcription in cardiac, as well as skeletal, myocytes, particularly the downstream CCAAT box-associated repeat. Thus, TGF-beta 1 and FGFs selectively induce an ensemble of 'fetal' genes and differentially regulate alpha-actin transcription in cardiac muscle cells.

Genetic dissection of cardiac growth control pathways

Cardiac muscle cells exhibit two related but distinct modes of growth that are highly regulated during development and disease. Cardiac myocytes rapidly proliferate during fetal life but exit the cell cycle irreversibly soon after birth, following which the predominant form of growth shifts from hyperplastic to hypertrophic. Much research has focused on identifying the candidate mitogens, hypertrophic agonists, and signaling pathways that mediate these processes in isolated cells. What drives the proliferative growth of embryonic myocardium in vivo and the mechanisms by which adult cardiac myocytes hypertrophy in vivo are less clear. Efforts to answer these questions have benefited from rapid progress made in techniques to manipulate the murine genome. Complementary technologies for gain- and loss-of-function now permit a mutational analysis of these growth control pathways in vivo in the intact heart. These studies have confirmed the importance of suspected pathways, have implicated unexpected pathways as well, and have led to new paradigms for the control of cardiac growth.

Expression of proto-oncogenes and gene mutation of sarcomeric proteins in patients with hypertrophic cardiomyopathy

Several mutations of cardiac beta-myosin heavy chain (beta-MHC) gene were reported in patients with hypertrophic cardiomyopathy (HCM). Involvement of proto-oncogenes has been shown in the mechanism of experimental cardiac hypertrophy. This study sought to examine the effects of c-H-ras and c-myc expression in the steady-state myocardium on hypertrophic changes and to evaluate the possible interaction between beta-MHC mutation and proto-oncogene expression in HCM. Endomyocardial biopsy was performed in 17 HCM patients (5 beta-MHC mutations and 1 troponin T mutation) and 7 control subjects (no mutation). Reverse transcription-polymerase chain reaction analysis revealed c-H-ras expression in all members of both groups. Cardiomyocyte size was correlated with the expression level of c-H-ras (P<0.001), and c-H-ras expression was upregulated in HCM patients (P<0.01). HCM patients with a beta-MHC mutation had the higher c-H-ras expression than did control subjects or patients without a mutation (P<0.01). c-myc mRNA was expressed in 7 of 17 HCM patients but not in control subjects. Myocyte size was greater in c-myc-positive HCM patients than in control subjects and c-myc-negative HCM patients (P<0.001 and P<0.05, respectively). The proto-oncogene expression did not affect clinical findings, myocardial fibrosis, or disarray. In conclusion, c-H-ras and c-myc expression in the steady-state myocardium may play a role in the hypertrophic mechanism in HCM. It is possible that ss-MHC gene mutation has some effect on the regulation of proto-oncogene expression in HCM.

C-myc is expressed in mouse skeletal muscle nuclei during post-natal maturation

Previous data have indicated the presence of c-myc mRNA and protein in mature skeletal muscle, but whether the protein is present as a Myc/Max heterodimer capable of influencing transcription in that tissue or its potential role in pathological muscle tissue has not been examined. The expression of c-myc in normal and mdx dystrophic mouse skeletal muscle was therefore investigated. C-myc mRNA was detected by Northern hybridisation in normal and dystrophic mouse muscle from mice up to 40 days of age and immunohistochemical staining confirmed that c-myc protein was expressed in muscle fibre nuclei in the muscles of mice up to 40 days of age. The presence of Myc-containing complexes that are able to bind to the concensus Myc/Max binding site was demonstrated in these muscles using the electrophoretic mobility shift assay. These results show that c-myc protein is expressed in functional complexes in both normal and dystrophic mouse skeletal muscle during post-natal maturation. They also show that expression of c-myc in mouse skeletal muscle decreases to undetectable levels by about 40 days of age. Although no differences were detected between the expression of c-myc in mdx and control mouse muscle, these data show that muscle contains Myc protein which has previously been demonstrated to be capable of initiating programmed cell death in other tissues.

Hypertrophy, pathology, and molecular markers of cardiac pathogenesis

Increased ventricular expression of several genes, including atrial natriuretic factor (ANF), has been documented in experimental models of cardiac hypertrophy. It remains to be clarified whether altered expression of these genes is a consistent marker of the hypertrophy itself or a marker of some parallel pathogenetic process. Using a transgenic mouse model of hypertrophic cardiomyopathy as a tool, we assessed the relationship between the amount of ventricular ANF gene expression and the degree of hypertrophy as well as the relationship between the cells expressing ANF and tissue pathology. We determined that hypertrophy is not always associated with increased ventricular expression of ANF and that cells expressing ANF are found in regions of tissue pathology. We propose that alteration in the ventricular expression of this gene is a sensitive indicator of cardiac pathogenesis and may result from a number of different stimuli that include, among others, abnormal tissue architecture and hemodynamic load.

Molecular and cellular prospects for repair, augmentation, and replacement of the failing heart

Molecular and cellular biology offer the promise of new approaches to the treatment of heart failure. This article discusses the basic science background, the current state of investigation, and the potential for therapeutic application of these new sciences. It also emphasizes the limitations and unknowns in this frontier. Three approaches are presented: First, increasing the number of myocytes in the heart, previously held to be untenable because postnatal cardiomyocytes do not divide, may be possible by regulating the cell cycle to reinduce cardiac growth. Also, nonmyocytes extant in the heart may be coaxed into differentiating into cardiomyocytes, or exogenous muscle cells may be grafted into the myocardium. Second, cardiac function may be augmented by molecular therapies that increase contractile protein function or regulate beta-adrenergic receptors or Ca++ channels. Third, improved prospects for transplantation of the failed heart may occur by genetic modification of a xenograft donor heart that reduces the chance of immune rejection by the human recipient. The formulation for the successful application of any of these therapies depends on not only the creativity of scientists but also the wisdom of physicians.

Regulation of rat cardiac myocyte growth by a neuronal factor secreted by PC12 cells

Sympathetic innervation of cardiac myocytes in vitro induces growth independent of anatomic contact between the neurons and myocytes and is not mediated by alpha- or beta-adrenergic receptor stimulation. To establish a model system that will allow purification and identification of the neuronal factor(s) responsible for mediating this regulation, we have initiated studies utilizing conditioned medium from the PC12 cell line. PC12 cells acquire a cholinergic sympathetic neuronal phenotype when exposed to nerve growth factor. Culture medium conditioned by neuronal PC12 cells, but not nonneuronal PC12 cells, induces growth in newborn rat cardiac myocytes as measured by surface area and [35S]methionine incorporation into protein and increases expression of atrionatriuretic peptide, a marker for myocyte hypertrophy. The magnitude of the growth response is dose-dependent and mimics the response to sympathetic innervation. The myocyte response to conditioned medium is not detectable after 24 h of exposure; maximal rate of protein synthesis is obtained within 48 h. Neuronally differentiated PC12 cell-conditioned medium stimulation of growth could not be mimicked by alpha- or beta-adrenergic agonists or muscarinic agonists, nor inhibited by alpha- or beta-adrenergic antagonists, nor by muscarinic antagonists. Neuropeptide Y and somatostatin, peptides known to be present in PC12 cells and sympathetic neurons, were also ineffective at reproducing the effect of neuronally differentiated PC12 cell-conditioned medium. These data indicate that neuronal cells release a soluble factor, different from neurotransmitter, which stimulates myocyte growth. They further identify the PC12 cell line as providing a convenient and abundant supply of this molecule, thus facilitating its further characterization.

Immunocytochemical detection of enhanced expression of c-myc protein in the heart of cardiomyopathic hamster

An immunocytochemical study was performed to examine the expression of cellular c-myc protein in the heart of 30-, 120- and 180-day-old cardiomyopathic Syrian UM-X7.1 hamsters. The heart of age- and sex-matched BIO-RB hamster was used as normal control. In paraffin sections, an immunostaining for c-myc was markedly increased in cytoplasm of cells from the UM-X7.1 heart as compared with that of the BIO-RB heart which showed a weak staining. However, c-myc was localized in nuclei of cells in frozen sections of the heart. Specific cell types of the heart were differentiated with anti-vimentin, and we found that the increased expression of c-myc was present in nuclei of muscle cells of the UM-X7.1 myocardium. A quantitative study of c-myc-positive nuclei of muscle and nonmuscle cells was carried out by a video micrometer. The mean number of c-myc-positive nuclei of muscle cells was significantly higher in the cardiomyopathic heart than in the control heart from hamsters of all ages studied. These results suggest that the increase of c-myc protein may relate to the pathological state or pathogenesis of the hereditary cardiomyopathy.

Molecular aspects of mechanical stress-induced cardiac hypertrophy

To elucidate the signal transduction pathway from external stimuli to nuclear gene expression in mechanical stress-induced cardiac hypertrophy, we examined the time course of activation of protein kinases such as Raf-1 kinase (Raf-1), mitogen-activated protein kinase kinase (MAPKK), MAP kinases (MAPKs) and 90-kDa ribosomal S6 kinase (p90rsk) in neonatal rat cardiomyocytes. Mechanical stretch rapidly activated Raf-1 and its maximal activation was observed at 1-2 min after stretch. The activity of MAPKK was also increased by stretch, with a peak at 5 min after stretch. In addition, MAPKs and p90rsk were maximally activated at 8 min and at 10-30 min after stretch, respectively. Next, the relationship between mechanical stress-induced hypertrophy and the cardiac renin-angiotensin system was investigated. When the stretch-conditioned culture medium was transferred to the culture dish of non-stretched cardiac myocytes, the medium activated MAPK activity slightly but significantly, and the activation was completely blocked by the type 1 angiotensin II receptor antagonist, CV-11974. However, activation of Raf-1 and MAPKs provoked by stretching cardiomyocytes was only partially suppressed by pretreatment with CV-11974. These results suggest that mechanical stress activates the protein kinase cascade of phosphorylation in cardiac myocytes in the order of Raf-1, MAPKK, MAPKs and p90rsk, and that angiotensin II, which is secreted from stretched myocytes, activates a part of these protein kinases.

Na, K-ATPase isoform gene expression in normal and hypertrophied dog heart

OBJECTIVES

The catalytic alpha subunit of the sodium-potassium ATPase, the target of digitalis glycosides, has three isoforms; the expression of these isoforms is tissue-specific and developmentally regulated. While the effect of pressure overload on Na, K-ATPase isoform expression has been studied in rodent heart, there are no systematic data on this question in hearts of larger animals, which differ from those of rodents both in isoform composition and in glycoside sensitivity. Thus, we investigated the expression of Na, K-ATPase isoforms in normal dog heart; we also examined the effect of experimental left ventricular hypertrophy on isoform expression.

METHODS

hypertrophy was produced by aortic banding. Expression was assessed by quantitative Northern and Western blotting, immunofluorescence, and 3H-ouabain binding.

RESULTS

RNA blotting indicated that the alpha 3 isoform represented 11% of Na, K-ATPase mRNA in normal dog LV. Normal dog LV expressed alpha 1 and alpha 3 protein, but no detectable alpha 2; immunoreactive alpha 1 and alpha 3 protein were also present in Purkinje fibers. There was a statistically significant decrease in total expression of all alpha isoform mRNA's in hypertrophied dog LV, resulting in a greater proportion of alpha 1. The expression level of the alpha 3 isoform mRNA and protein was lower in hypertrophied hearts.

CONCLUSIONS

These results indicate a greater proportion of alpha 1 isoform pumps in experimental canine hypertrophy. Thus, shifts in NA, K-ATPase isoforms occur in pressure-overloaded heart in large animals as well as rodents.

Molecular remodelling in hypertrophied hearts from polyomavirus large T-antigen transgenic mice

Polyomavirus large T-antigen transgenic mice develop cardiac hypertrophy characterized by an increase in atrial natriuretic factor and beta-myosin heavy chain isoform expression. The aim of this study was to examine changes in proto-oncogene expression in hypertrophied hearts from the transgenic mice. Expression of early growth response-1 (Egr-1) mRNA was detected in hearts from all 15 transgenic mice, but was not detectable in 13 control mice. Reverse transcriptase-polymerase chain reaction experiments using Egr-1-specific primers confirmed the increase in Egr-1 mRNA in enlarged hearts from the transgenic mice. Expression of c-jun, junD and Ha-ras mRNAs was increased in the transgenic hearts 3, 17 and 2.8-fold respectively. Western blots showed an increase in c-myc, c-jun and ras protein in hypertrophied transgenic hearts. Immunofluorescence analyses confirmed an increase in Egr-1 and c-jun protein in transgenic cardiomyocytes. Proliferating cell nuclear antigen, Ki-ras and HSP 90 mRNAs were decreased 22, 2.7 and 3-fold, respectively in the transgenic hearts. Not altered in most hypertrophied hearts was expression of c-fos, junB, p53, c-neu, c-myc, HSP70, HSP27, TGF-beta or IGF 1 mRNAs. Proto-oncogene and growth factor gene expression in hypertrophy induced by PVLT expression is modulated with some proto-oncogenes increased and others decreased in expression.

Cell death in the failing heart: role of an unnatural growth response to overload

Hypertrophy of the overloaded heart, characterized by an increased number of sarcomeres, provides an adaptive, short-term response. However, when cardiac overload is long-standing, the hypertrophic response appears to cause shortened myocyte survival. The mechanisms responsible for the deleterious effects of chronic myocardial hypertrophy may include a maladaptive growth response of the mature heart. Because terminally differentiated adult cardiac myocytes have little or no capacity to divide, stimuli that promote growth in the overloaded adult heart cannot lead to normal cell division. Instead, overload initiates an unnatural growth response that appears to shorten cardiac myocyte survival, possibly because the same growth factors that mediate the hypertrophic response of the adult heart can also induce programmed cell death (apoptosis). The converting enzyme inhibitors and nitrates, which have growth-inhibitory as well as vasodilator effects, may improve prognosis in heart failure by inhibiting the production of transcription factors. These transcription factors stimulate both the unnatural growth response to overload and stimuli that lead to apoptosis. Since both beta-adrenergic agonists and cytokines, such as tumor necrosis factor-alpha, can stimulate production of similar transcription factors, evidence suggests that beta blockers and vesnarinone improve the prognosis in patients with heart failure possibly because of their ability to inhibit maladaptive growth.

Mechanical stress activates protein kinase cascade of phosphorylation in neonatal rat cardiac myocytes

We have previously shown that stretching cardiac myocytes evokes activation of protein kinase C (PKC), mitogen-activated protein kinases (MAPKs), and 90-kD ribosomal S6 kinase (p90rsk). To clarify the signal transduction pathways from external mechanical stress to nuclear gene expression in stretch-induced cardiac hypertrophy, we have elucidated protein kinase cascade of phosphorylation by examining the time course of activation of MAP kinase kinase kinases (MAPKKKs), MAP kinase kinase (MAPKK), MAPKs, and p90rsk in neonatal rat cardiac myocytes. Mechanical stretch transiently increased the activity of MAPKKKs. An increase in MAPKKKs activity was first detected at 1 min and maximal activation was observed at 2 min after stretch. The activity of MAPKK was increased by stretch from 1-2 min, with a peak at 5 min after stretch. In addition, MAPKs and p90rsk were maximally activated at 8 min and at 10 approximately 30 min after stretch, respectively. Raf-1 kinase (Raf-1) and (MAPK/extracellular signal-regulated kinase) kinase kinase (MEKK), both of which have MAPKKK activity, were also activated by stretching cardiac myocytes for 2 min. The angiotensin II receptor antagonist partially suppressed activation of Raf-1 and MAPKs by stretch. The stretch-induced hypertrophic responses such as activation of Raf-1 and MAPKs and an increase in amino acid uptake was partially dependent on PKC, while a PKC inhibitor completely abolished MAPK activation by angiotensin II. These results suggest that mechanical stress activates the protein kinase cascade of phosphorylation in cardiac myocytes in the order of Raf-1 and MEKK, MAPKK, MAPKs and p90rsk, and that angiotensin II, which may be secreted from stretched myocytes, may be partly involved in stretch-induced hypertrophic responses by activating PKC.

Myocardial expression of a constitutively active alpha 1B-adrenergic receptor in transgenic mice induces cardiac hypertrophy

Transgenic mice were generated by using the alpha-myosin heavy chain promoter coupled to the coding sequence of a constitutively active mutant alpha 1B-adrenergic receptor (AR). These transgenic animals demonstrated cardiac-specific expression of this alpha 1-AR with resultant activation of phospholipase C as shown by increased myocardial diacylglycerol content. A phenotype consistent with cardiac hypertrophy developed in adult transgenic mice with increased heart/body weight ratios, myocyte cross-sectional areas, and ventricular atrial natriuretic factor mRNA levels relative to nontransgenic controls. These transgenic animals may provide insight into the biochemical triggers that induce hypertrophy in cardiac disease and serve as a convenient experimental model for studies of this condition.

Transforming growth factor-beta 1 potentiated alpha 1-adrenergic and stretch-induced c-fos mRNA expression in rat myocardial cells

Since transforming growth factor-beta 1 (TGF-beta 1) has been recently shown to be expressed in the heart by mechanical stretch and ischemic injury, we examined the modulation of c-fos mRNA expression and amino acid uptake by TGF-beta 1 in rat myocardial cells. Pretreatment with TGF-beta 1 potentiated norepinephrine (NE)-induced and stretch-induced (+10% and +20% elongation, for 30 minutes) c-fos mRNA expression by 2.2-fold, whereas TGF-beta 1 alone did not induce c-fos mRNA expression in Northern blot analysis. NE-induced [14C]phenylalanine uptake was also potentiated with TGF-beta 1 pretreatment. The effect of TGF-beta 1 on the NE action was not blocked by propranolol but by prazosin. The protein kinase C activators (12-O-tetradecanoylphorbol 13-acetate [TPA], phorbol 12,13-dibutyrate, and 1-oleyl-2-acetyl-rac-glycerol) induced c-fos mRNA expression, which was also potentiated by TGF-beta 1. Cycloheximide (protein synthesis inhibitor) could not suppress the TGF-beta 1 actions. In nonmuscle cells, TGF-beta 1 modified neither adrenergic nor TPA-induced c-fos mRNA expression. These data suggested that TGF-beta 1 potentiated the c-fos mRNA expression and amino acid incorporation by modification of the alpha 1-adrenergic and stretch-activated protein kinase C pathway. This mechanism did not require protein synthesis.

The cAMP response element binding protein is expressed and phosphorylated in cardiac myocytes

Cardiac cells grow in response to a number of stimuli that activate intracellular signaling pathways. The cAMP-signaling pathway mediates the activation of gene transcription in other cell types by the cAMP response element binding protein (CREB-P). Our aim was to explore the physiological role of CREB-P in response to elevated cAMP in cardiac cells by determining if phosphorylation of CREB-P (to phosphoCREB-P) rapidly induces transcription in culture. Primary embryonic chick heart cultures were used in which cAMP was raised by forskolin (5 mumol/L) or isoproterenol (10 mumol/L) treatment. Since both these agents have inotropic effects, tension production was controlled with 2,3-butanedione monoxime (BDM). This allowed us to determine whether the cAMP-signaling pathway or the contractile state was regulating phosphorylation and transcription. The responses for time periods up to 2 hours were assayed with antibodies to detect phosphoCREB-P and by quantitative filter hybridization for creb gene expression. The staining intensity of the phosphoprotein increased in myocyte nuclei after 10 minutes and persisted for 1 hour with either forskolin or isoproterenol treatment. An increase in creb mRNA abundance was also detected, with the maximum level of expression being at 1 hour with forskolin treatment. These changes are independent of the contractile state, because BDM itself caused no change. BDM plus forskolin induced the same pattern of creb expression as observed with forskolin alone. Therefore, we conclude that elevation of cAMP leads to phosphorylation of CREB-P and an increase in creb mRNA abundance.

Localization of c-myc protooncogene expression in the rat heart in vivo and in the isolated, perfused heart following treatment with norepinephrine

We have investigated the expression of the protooncogene c-myc in rat hearts following exposure to norepinephrine, both in vivo and in isolated perfused preparations. Both chronic and acute norepinephrine treatment produced a rapid, transient elevation of c-myc mRNA in adult rat hearts, but chronic infusion produced a second, larger increase. This expression profile was characteristic for c-myc since it was not found for four other protooncogenes. In the isolated, perfused heart, addition of norepinephrine to the perfusion buffer and elevation of perfusion pressure separately increase c-myc mRNA suggesting both direct hormonal and hemodynamic factors might be important in vivo. Immunocytochemistry showed that Myc protein accumulated predominantly in the nuclei of non-myocyte cells following norepinephrine treatment indicating that expression at the mRNA level culminated in protein synthesis. These findings suggest that the c-myc expression observed in the hypertrophying adult heart following exposure to norepinephrine may be associated with proliferating cells like fibroblasts rather than cardiomyocytes.

c-myc gene expression is localized to the myocyte following hemodynamic overload in vivo

Expression of the proto-oncogene c-myc increases in the hemodynamically overloaded heart, but expression by cardiac myocytes has not been shown. To address this issue, right ventricular overload was induced in cats by pulmonary artery banding. Expression of c-myc and alpha-skeletal actin mRNA were determined by Northern analysis. Immuno-reactive Myc protein was identified by histochemical staining. Steady state levels of c-myc mRNA peaked within 2 h after banding. Levels of alpha-skeletal actin mRNA were maximally increased 48 h-1 week after banding and were still elevated at 1 month. Prominent staining of myocyte nuclei for immunoreactive Myc protein was detected 48 h after banding although a few interstitial nuclei were also positive. These studies show that c-myc and alpha-skeletal actin gene expression are upregulated in a large animal model of hemodynamic overload. The localization of the immunoreactive Myc protein to right ventricular myocyte nuclei after pulmonary artery banding supports the hypothesis that c-myc induction is part of a general response in cardiac hypertrophy that is common to many mammalian species.

Increased expression of atrial myosin light chain 1 in the overloaded human left ventricle: possible expression of fetal type myocytes

We examined the isoforms of myosin light chain 1 in the human left ventricles using pyrophosphate and sodium dodecyl sulfate polyacrylamide gel electrophoresis, peptide mapping, and immunoblotting with monoclonal antibodies against human atrial light chain 1. The relationship between hemodynamic parameters and light chain 1 isoform composition was compared among groups of patients with hypertrophic cardiomyopathy (n = 8), dilated cardiomyopathy (n = 9) and aortic stenosis (n = 5), and controls (n = 6). (1) The light chain 1, which differed from ventricular light chain 1 found in the normal adult ventricle, was highly expressed in the overload left ventricle, and was identical to atrial and fetal ventricular light chain 1 with respect to the physiochemical and immunological properties. (2) The expression of atrial/fetal light chain 1 was augmented in the subendocardial area in comparison with the mid- or subepicardial areas in the hypertrophied left ventricles. (3) The values (%) of the relative expression of atrial/fetal light chain 1 to total light chains 1 determined by densitometric analysis were significantly higher in patients with dilated cardiomyopathy (40.2 +/- 5.8) and those with aortic stenosis (43.1 +/- 6.2) than in the controls (16.9 +/- 2.5) (p less than 0.01), but there was no significant difference between the patients with hypertrophic cardiomyopathy (28.0 +/- 3.7) and the controls. (4) The values of the ratio significantly correlated with those of peak circumferential wall stress (r = 0.53, p less than 0.005). These results suggest that atrial/fetal light chain 1 is expressed in the left ventricles in response to the increased hemodynamic load.

Pretranslational regulation of two cardiac glucose transporters in rats exposed to hypobaric hypoxia

To investigate the mechanism by which cardiac glucose utilization increases during hypoxia and increased work load, we studied the effect of 2 and 14 days of hypobaric hypoxia on the expression of two subtypes of the facilitative D-glucose transporter, the GLUT-4 or "insulin-regulatable" isoform and the GLUT-1 isoform thought to mediate basal transport. Rats lose weight when exposed to hypobaric hypoxia, so fasting controls were used in the 2-day studies and pair-fed controls in the 14-day experiments. Hypobaric hypoxia (PO2 69 mmHg) resulted in right ventricular (RV), but not left ventricular (LV), hypertrophy. RV and LV GLUT-1 mRNA levels increased 2- to 3-fold after 2 days and 1.5- to 2-fold after 14 days of hypobaric hypoxia compared with both fasted rats and normal controls. RV GLUT-1 protein increased approximately 3-fold and LV GLUT-1 protein increased 1.5-fold after 14 days of hypobaric hypoxia vs. both pair-fed and normal controls. RV GLUT-4 mRNA decreased to 26% and RV GLUT-4 protein decreased to 54% of normal control levels as a result of 2 days of hypobaric hypoxia. RV GLUT-4 mRNA decreased to 64% of normal control levels with no change in RV GLUT-4 protein as a result of 2 days of fasting. We conclude that hypobaric hypoxia increases cardiac GLUT-1 expression at the pretranslational level in both ventricles. The greater increase in GLUT-1 protein on the right suggests an additive effect of pressure overload. GLUT-4 expression is reduced early in the development of RV hypertrophy.

Growth factors, growth factor response elements, and the cardiac phenotype

Fibroblast growth factors (FGF) and type beta-1 transforming growth factor (TGF beta 1) are pleiotropic regulatory peptides which are expressed in myocardium in a precise developmental and spatial program and are up-regulated, in the adult heart, by ischemia or a hemodynamic burden. The accumulation of trophic factors after aortic banding supports the hypothesis that autocrine or paracrine pathways might function to mediate, in part, the consequences of mechanical load. Our laboratory has demonstrated that cardiac muscle cells are targets for the action of peptide growth factors and, more specifically, that modulation of the cardiac phenotype by basic FGF (bFGF) and TGF beta 1 strongly resembles the induction of fetal cardiac genes--including skeletal alpha-actin (SkA), beta-myosin heavy chain, and atrial natriuretic factor--which are characteristic of pressure-overload hypertrophy. Unexpectedly, and despite effects like those of bFGF on five other cardiac genes, acidic FGF (aFGF) was found to repress, rather than stimulate, SkA transcription in neonatal cardiac muscle cells. The proximal 200 nucleotides of a heterologous SkA promoter were sufficient for basal tissue-specific transcription, for induction by bFGF, and for inhibition by aFGF. Thus, both positive and negative regulation by peptide growth factors can be localized to the proximal SkA promoter. Full promoter activity required each of three CC[A/T]6GG motifs similar to the serum response element (SRE) for activation of the c-fos proto-oncogene, as previously shown for SkA transcription in a skeletal muscle background. The most proximal SRE, SRE1, was sufficient in the absence of other SkA promoter sequences for efficient tissue-specific expression in cardiac myocytes (versus cardiac fibroblasts), and was stimulated by bFGF to the same extent as the full-length promoter and endogenous gene. Despite its ability to repress the SkA promoter, aFGF had no significant effect on SRE1. Both FGFs up-regulated the canonical fos SRE, to a comparable degree. Thus, SRE1 can discriminate between signals generated in cardiac myocytes by bFGF and aFGF. In cardiac myocyte extracts, two predominant proteins contact SRE1: serum response factor (SRF) and a second protein, F-ACT-1. Thus, serum response factor and F-ACT-1 are candidate trans-acting factors for basal transcription of the SkA gene in cardiac muscle cells and for induction of SkA by bFGF and, potentially, other trophic signals.

Biological adaptation of the myocardium to a permanent change in loading conditions

Cardiac hypertrophy due to permanent mechanical overloading is only one example among thousands of the general process of biological adaptation. The process is randomly governed and results in at least one thermodynamical benefit: to be adaptational and to induce several changes in gene expression. Some of these changes are detrimental, some can even be useless. The cascade of events which finally leads to a permanent modification of the genetic expression involves an initial signal, likely to be the stretch, a pathway which transducts the signal, and a transient change in genetic expression which transmits competence to the cell to be transformed. The permanent modifications occur at all cellular levels including the sarcomere, sarcolemma, energy metabolism, and extra-cellular matrix, but they are species-specific and differ in the ventricles and the atria.

Expression of c-myc and c-fos in rat skeletal muscle. Evidence for increased levels of c-myc mRNA during hypertrophy

The levels of c-myc and c-fos mRNA were investigated in rat skeletal muscle by Northern hybridization. During post-natal development in the rat, c-myc mRNA levels were similar at birth and at 7 and 21 days of age, but then declined at 90 days and were barely detectable at 1 year. c-fos mRNA levels followed this pattern of expression until 90 days, but showed a large increase at 1 year. Hypertrophy of soleus and plantaris muscles was induced either by severance of the tendon to the synergistic gastrocnemius (tenotomy) or by administration of the beta-adrenoceptor agonist clenbuterol. In both cases hypertrophy was associated with a rapid increase in c-myc mRNA levels. Following tenotomy the increase was both greater (8-fold) and more rapid (3 h) in soleus than in plantaris (2-3 fold, 12 h). Similar effects were observed during clenbuterol administration. Neither treatment caused any alteration in c-fos mRNA levels in the plantaris muscle. The results show that increased c-myc mRNA levels are an early event in the response of skeletal muscle to hypertrophic stimuli; it is argued that this occurs within the differentiated skeletal muscle fibres.

Localization and regulation of c-fos and c-jun protooncogene induction by systolic wall stress in normal and hypertrophied rat hearts

The effect of changes in left ventricular (LV) systolic force generation on cardiac c-fos and c-jun protooncogene expression was studied by using isolated beating hearts from male Wistar rats. An isovolumic buffer-perfused heart preparation was utilized in which coronary flow and heart rate were held constant and increments in LV balloon volume were used to generate defined levels of LV systolic wall stress. Using Northern and slot-blot analyses, we found that LV tissue from control hearts that generated high levels of LV systolic wall stress expressed 3- to 4.4-fold higher c-fos and c-jun mRNA levels in comparison with tissue from the respective flaccid right ventricles, and in comparison with LV tissue from hearts that generated minimal LV systolic wall stress. To distinguish the role of passive LV diastolic wall stretch from active LV force generation, we found that distension of the LV balloon per se did not have a significant effect on protooncogene induction in hearts perfused with 2,3-butanedione monoxime, which prevents systolic cross-bridge cycling and force generation. In additional hearts studied at a constant LV balloon volume to generate an LV end-diastolic pressure of 10 mm Hg, c-fos mRNA levels were proportional to the magnitude of peak LV systolic wall stress (r = 0.823, P less than 0.05). In these protocols, Fos protein was localized by immunohistochemistry in myocyte nuclei with minimal staining in fibroblasts and vascular smooth muscle. When c-fos and c-jun mRNA expression was compared in hearts with chronic LV hypertrophy due to ascending aortic banding and age-matched control hearts that generated similar incremental levels of LV systolic wall stress, significantly lower levels of c-fos and c-jun mRNA were measured in the hypertrophied hearts. However, there was no difference in protooncogene mRNA expression in response to stimulation by the Ca2+ ionophore A23187. These data suggest that, in this isolated isovolumic beating heart preparation, the active generation of an acute increment in LV systolic force independent of passive diastolic myocardial stretch causes a rapid induction of both c-fos and c-jun, which is down-regulated in the presence of established LV hypertrophy.

The vascular smooth muscle alpha-actin gene is reactivated during cardiac hypertrophy provoked by load

Cardiac hypertrophy triggered by mechanical load possesses features in common with growth factor signal transduction. A hemodynamic load provokes rapid expression of the growth factor-inducible nuclear oncogene, c-fos, and certain peptide growth factors specifically stimulate the "fetal" cardiac genes associated with hypertrophy, even in the absence of load. These include the gene encoding vascular smooth muscle alpha-actin, the earliest alpha-actin expressed during cardiac myogenesis; however, it is not known whether reactivation of the smooth muscle alpha-actin gene occurs in ventricular hypertrophy. We therefore investigated myocardial expression of the smooth muscle alpha-actin gene after hemodynamic overload. Smooth muscle alpha-actin mRNA was discernible 24 h after coarctation and was persistently expressed for up to 30 d. In hypertrophied hearts, the prevalence of smooth muscle alpha-actin gene induction was 0.909, versus 0.545 for skeletal muscle alpha-actin (P less than 0.05). Ventricular mass after 2 d or more of aortic constriction was more highly correlated with smooth muscle alpha-actin gene activation (r = 0.852; P = 0.0001) than with skeletal muscle alpha-actin (r = 0.532; P = 0.009); P less than 0.0005 for the difference in the correlation coefficients. Thus, smooth muscle alpha-actin is a molecular marker of the presence and extent of pressure-overload hypertrophy, whose correlation with cardiac growth at least equals that of skeletal alpha-actin. Induction of smooth muscle alpha-actin was delayed and sustained after aortic constriction, whereas the nuclear oncogenes c-jun and junB were expressed rapidly and transiently, providing potential dimerization partners for transcriptional control by c-fos.

Segregation of atrial-specific and inducible expression of an atrial natriuretic factor transgene in an in vivo murine model of cardiac hypertrophy

To study the mechanisms that activate expression of the atrial natriuretic factor (ANF) gene during pressure-induced hypertrophy, we have developed and characterized an in vivo murine model of myocardial cell hypertrophy. We employed microsurgical techniques to produce a stable 35- to 45-mmHg pressure gradient across the thoracic aorta of the mouse that is associated with rapid and transient expression of an immediate-early gene program (c-fos/c-jun/junB/Egr-1/nur-77), an increase in heart weight/body weight ratio, and up-regulation of the endogenous ANF gene. These responses that are identical to those in cultured cell and other in vivo models of hypertrophy. To determine whether tissue-specific and inducible expression of the ANF gene can be segregated, we used a transgenic mouse line in which 500 base pairs of the human ANF promoter region directs atrial-specific expression of the simian virus 40 large tumor antigen (T antigen), with no detectable expression in the ventricles. Thoracic aortic banding of these mice led to a 20-fold increase in the endogenous ANF mRNA in the ventricle but no detectable expression of the T-antigen marker gene. This result provides evidence that atrial-specific and inducible expression of the ANF gene can be segregated, suggesting that a distinct set of regulatory cis sequences may mediate the up-regulation of the ANF gene during in vivo pressure overload hypertrophy. This murine model demonstrates the utility of microsurgical techniques to study in vivo cardiac physiology in transgenic mice and should allow the application of genetic approaches to identify the mechanisms that activate ventricular expression of the ANF gene during in vivo hypertrophy.

Cardiac growth factors

The role of polypeptide growth factors in cardiovascular ontogeny, function, and pathologic states is poorly understood. Recent investigations demonstrate that the myocardium produces both known and novel growth factors, which are highly regulated during development and disease, and have suggested that peptide growth factors may direct cardiac organogenesis and adaptation. Aspects of growth factor production, transduction, and action in myocardium are distinct to the cardiac muscle lineage and were not foreseen from results in simpler systems. Transforming growth factor beta 1 and fibroblast growth factors (FGFs) selectively up-regulate an ensemble of tissue-specific genes associated with the fetal myocardium. One of these, encoding the skeletal muscle isoform of alpha-actin, is activated by basic FGF yet is inhibited by acidic FGF. A serum response element of this gene is selectively induced, in cardiac myocytes, by basic FGF but not acidic FGF. Thus, cardiac muscle is an especially intriguing model for the analysis of growth factor signalling pathways that control differentiated gene transcription.